2019 EJRNL PP BINBIN WANG 1.pdf
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Terbatas Ratnasari
» ITB
Terbatas Ratnasari
» ITB
This paper presents the experimental results from an air bubble-in-chain induced flow
at different bubble release frequencies, resulting in a range of 1510–2318 for the Reynolds
number based on bubble parameters. Mean flow and turbulent statistics were measured
using planar particle image velocimetry. The bubble rise velocities were validated to be the
superposition of the mean vertical velocity of water and the terminal velocity of the isolated
bubbles. We tested the scaling parameters for the bubble-in-chain induced flow using
those typically for bubble plumes, which demonstrates the intrinsic similarities of these
two bubbly flows. We found that the radial profiles of normalized mean vertical velocity
for bubble-in-chain induced flows and bubble plumes collapse well using the center-line
velocity and the half-width of the velocity profile.We found the half-width correlated with
the characteristic length scales of the bubble wakes. The normalized Reynolds stresses
using the local velocity square show the same shape of profiles for both bubbly flows,
but they do not collapse onto universal curves. The normalized turbulent kinetic energy is
a function of normalized gas flow rate and approaches an asymptotic value of 0.05. The
dynamic length scale, a function of initial kinematic buoyancy flux, was found to govern
turbulent dissipation rates in both bubbly flows. The normalized one-dimensional velocity
energy spectra collapse in the regime of kv? > 0.1 where kv is the wave number and ? is
the Kolmogorov length scale, showing a ?3 slope as the result of energy balance between
production from bubble motion and dissipation in the bubble wakes. The wake length scale
was found to define the lower-end wave number of the ?3 slope.